1. Field of the Invention
The present invention relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus for scanning planes constituting image bearing members with plural laser beams, adapted for use in an image recording apparatus such as a color laser beam, a multi-color laser beam printer and the like printer utilizing an electrophotographic process.
2. Related Background Art
In the optical scanning apparatus for example of a color laser beam printer (color LBP), there has already been employed image recording by scanning photosensitive image bearing members with plural laser beams.
In such conventional apparatus, plural laser beams are respectively introduced to different reflective deflecting planes of a rotary polygon mirror constituting a light deflector, and an f-.theta. lens is provided for each of the laser beams reflected by said reflective deflecting planes. The optical scanning of the image bearing plane is achieved by the laser beam from the f-.theta. lens, utilizing an anamorphic plane corrected for the inclination of the reflective deflecting plane. However such conventional apparatus tends to become bulky and complex in structure, since a set of scanning optical system is provided for each of the laser beam.
On the other hand, Japanese Laid-Open Utility Model No. 61-92917 and Japanese Laid-Open Patent and No. 58-79215 propose a method of mixing two laser beams into one by utilizing two beams of different polarization characteristics or different wavelengths, focusing such united laser beam at a point close to the scanned plane utilizing the lens systems of a number equal to a half of the number of original laser beams, then separating each laser beam into two by a polarizing beam splitter or a dichroic mirror and scanning respective image bearing planes with thus separated laser beams.
However, in the above-mentioned method, because of the mixing and separation of two laser beams the entire apparatus inevitably becomes complex, and certain light leak is unavoidable depending on the incident angle characteristics of the polarizing beam splitter and the dichroic mirror at such mixing or separation if the optical scanning angle on the scanned plane becomes large. For this reason it has not been possible to increase the scanning angle of the light too much.
Also the U.S. Pat. No. 4,561,717 discloses a method, as shown in FIG. 1, of introducing plural laser beams into a single reflective deflecting plane 520a of a light deflector 520 composed of a rotary polygon mirror, obliquely with different angles to the perpendicular direction to the scanning direction of scanned planes 561, 562. The beams are condensed by an f-.theta. lens 530 composed of spherical faces, then separated into plural beams by an optical device such as mirrors 541, 542 distanced from the laser beams, and guided to image bearing members 561, 562 for effecting the optical scanning thereon.
In such case, the laser beam obliquely entering the f-.theta. lens 530 composed of spherical faces causes curvature of scanning line due to the optical characteristics of the f-.theta. lens. Conventionally such curvature has been corrected by cylindrical lenses 551, 552 positioned in front of the image bearing planes.
In such method, however, since the light beam enters the cylindrical lens with a scanning angle (image angle in the scanning direction), the apparent refractive power of the cylindrical lens increases with the increase in the scanning angle. Thus the laser beam is focused in front of the scanned plane, so that the curvature of image plane becomes stronger, and the diameter of the laser beam becomes different between the central area and peripheral area of the scanned frame.
Also, if three or more laser beams are employed in the apparatus, the f-.theta. lens of spherical faces receives at least two oblique incident angles. If the angles are mutually different, the obtained scanning lines becomes different depending on the characteristics of the f-.theta. lens.
More specifically, since a skewed ray is introduced to the f-.theta. lens 530 composed of spherical faces, the f-.theta. characteristics are distorted depending on the oblique incident angle.
The f-.theta. characteristics in the scanning direction do not vary linearly for the oblique incident light. For an incident angle .phi..sub.0, the coordinate X(.phi.=.phi..sub.0)(.theta.) in the scanning direction of the scanning beam is correlated with the scanning angle .theta. of the light deflector by: ##EQU1## wherein f is the focal length of the f-.theta. lens, and .alpha. is the incident angle of the skewed light beam.
Consequently the scanning lines are inevitably different for different incident angles to the f-.theta. lens. This phenomenon leads to mutual aberration of scanning lines of different colors for example in a color laser beam printer requiring precise registration of multiple colors. As an example, if a same second condenser lens (anamorphic lens) is employed for correction the curvature of two scanning lines respectively of oblique incident angles of 2.5.degree. and 7.5.degree., an f-.theta. lens of spherical faces with a focal length of 313.55 mm gives rise to a difference of 0.6 mm in the length of the scanning lines at a scanning angle of 30.degree. (a scanning position of 160 mm). Then, if the image magnification in the scanning direction is modified to match the scanning position at the scanning angle of 30.degree., there will result an aberration of about 60 .mu.m between the scanning lines at a scanning angle of about 16.degree..
Even if the scanning lines are positionally balanced, an aberration of .+-.30-40 .mu.m is unavoidable, corresponding to an aberration of a half of the pixel for example in a printer with a resolving power of 400 DPI.